This invention relates to an electronically switched linear motor producing a high specific thrust or push, possibly incorporating a position sensor, and which is particularly adapted to realize linear actuators of the high precision type and with wide operating range.
Many applications require the use of linear actuators. More particularly, in the fields of industrial automation, robotics, and machine tools control, the need is common for realizing the movements of linear actuation in a rigid and precise manner, with a device that is as fast and insensible to environment and load changes as possible.
According to the actual technique, in most applications the linear actuating movement is obtained either by means of a hydraulic or pneumatic cylinder -for applications requiring moderate speed and precision- or by means of an electric actuator in order to obtain the maximum of speed and control bandwidth. Such electric actuator is generally made up by a high performance servomotor (e.g. a D.C. or a brushless motor) that transmits the rotating motion through a torque limiting coupling device to a balls-circulating screw converting the rotation movement into a rectilinear motion. The system is completed by a position sensor in the motor axis for measuring the position of the actuated member. This embodiment does not allow for a high precision since the errors or the screw and of the associated bearings cannot be corrected.
According to an alternative embodiment, quite less frequent since more costly, the position sensor is of the linear type and is carried by the actuated member. Such solution is more precise, but is limited by tne transmission elasticity which generates a resonance frequency of the kynematic chain, as a function of the mass of the load, which sets an upper limit for the bandwidth of the control system.
The use of an electric linear motor would permit to eliminate the above mentioned drawbacks by making the system theoretically simpler and eliminating the elasricity and the resonance effects which are typical of the transmission. However this solution has been used only in a very limited number of applications by reason of the peculiar characteristics of the linear motors presently available which are characterized by:
(1) Very low thrust/mass and thrust/mass ratios which render the motors intrinsically not very dynamic;
(2) High parasitic thrust orthogonal to tne useful thrust, and usually equal to many times this letter; this phenomenon causes stresses and wear in the bearings and often requires the use of expensive support systems, such as fluidostatic bearings, thus causing often an undue expense and duplication of the normal linear supports of tne actuated slider;
(3) Very reduced air gaps, typically in the order of a few hundredths of millimeters, which make their manufacturing difficult and costly.
To the above listed disadvantages it is to be added that a linear position sensor, which too is very costly, is to be provided for when using a linear motor.